The invention relates to a picture display system including a colour display tube having a neck accommodating an electron gun assembly for generating three electron beams, and an electromagnetic deflection unit including
a field deflection coil of the saddle type having a front and a rear end for deflecting electron beams generated in the display tube in a vertical direction and
a line deflection coil of the saddle type likewise having a front and a rear end for deflecting electron beams generated in the display tube in a horizontal direction and yoke ring of ferromagnetic material surrounds the two deflection coils and has front and rear end faces extending transversely to the tube axis, the electron beam traversing the coils in the direction from the rear to the front ends when the deflection unit is arranged on a display tube.
FOr some time a colour display tube has become the vogue in which three electron beams are used in one plane; the type of such a cathode ray tube is sometimes referred to as "in-line". In this case, for decreasing convergence errors of the electron beams, a deflection unit is used having a line deflection coil generating a horizontal deflection field of the pincushion type and a field deflection coil generating a vertical deflection field of the barrel-shaped type.
Deflection units for in-line colour display tube systems can in principle be made to be entirely self-convergent, that is to say, in a design of the deflection unit which ensures convergence of the three electron beams on the axes, anisotropic y-astigmatism errors, if any, can simultaneously be made zero in the corners without this requiring extra correction means. While it would be interesting from a point of view of manufacture to have a deflection unit which is selfconvergent for a family of display tubes of the same deflection angle and neck diameter, but different screen formats, the problem exists, however, that a deflection unit of given main dimensions can only be used for display tubes of one screen format. This means that only one screen format can be found for a fixed maximum deflection angle in which aa given deflection unit is self-convergent without a compromise (for example, the use of extra correction means).
The Netherlands Patent Specification 174 198 provides a solution to this problem which is based on the fact that, starting from field and line deflection coils having given main dimensions, selfconvergent deflection units for a family of display tubes having different screen formats can be assembled by modifying the effective lengths of the field and line deflection coils with respect to each other. This solution is based on the recognition that, if selfconvergence on the axes has been reached, the possibly remaining anisotropic y-astigmatism error (particularly the y-convergence error halfway the diagonals) mainly depends on the distance between the line deflection point and the field deflection point and to a much smaller extent on the main dimensions of the deflection coils used. If deflection units for different screen formats are to be produced while using deflection coils having the same main dimensions, the distance between the line and field deflection points may be used as a parameter to achieve self-convergence for a family of display tubes having different screen formats but the same maximum deflection angle.
The variation in the distance between the line and field deflection points necessary for adaption to different screen formaats is achieved in the prior art by either decreasing or increasing the effective coil length of the line deflection coil or of the field deflection coil, or of both - but then in the opposite sense - with the maiin dimensions of the deflection coils remaining the same and with the dimensions of the yoke ring remaining the same, for example, by mechanically making the coil or coils on the rear side smaller and longer, respectively, by a few millimeters, or by positioning, with the coil length remaining the same, the coil window further or less far to the rear (so thata the turns on the rear side are more or less compressed). To achieve this, saddle-shaped line and field deflection coils of the shell type were used. These are coils having ends following the contour of the neck of the tube at least on the gun side. This is in contrast to the conventional saddle coils in which the gun-sided ends, likewise as the screen-sided ends, are flanged and extend transversely to the tube surface. When using saddle coils of the shell type it is possible for the field deflection coil (and hence the vertical deflection field) to extend further to the electron gun assembly than the line deflection coil, if the field design so requires. However, there are also deflection units with deflection coils of the conventional saddle type, which means that - as stated - they have front and rear ends located in planes extending at an angle (generally of 90.degree. ) to the tube axis. (A special type of such a deflection unit with conventional saddle coils is, for example, the deflection unit described in EP 102 658 with field and line deflection coils directly wound on a support). In this case it has until now been impossible to extend the vertical deflection field further to the electron gun assembly than the horizontal deflection field, because the field deflection coil is enclosed between the flanges of the line deflection coil.